Semipermeable membrane technology has seen increased application in recent years. Notable uses are in providing potable water from seawater, fractionating solutions containing macromolecular components, controlling the rate of drug release into the body, and removing urea and other toxins from the blood, i.e., with an artificial kidney.
The term "semipermeable membrane" implies that certain materials can pass through the membrane, while others cannot. Membranes and the processes which employ them are generally classified by the particle sizes of the materials which are retained by the membranes in passing a fluid through them.
Particles having a mean diameter greater than about 1 micron can be separated from a liquid carrier using gravity filtration through ordinary filter paper. On the other hand, particles smaller than about 10.sup.-3 microns, i.e., about 1-10 Angstroms (the size of simple anions and cations) can be separated from a liquid by reverse osmosis or "RO". Ultrafiltration separates particles larger than about 10.sup.-2 microns. In practical terms, proteins and viruses can be separated from an aqueous carrier by ultrafiltration.
Nanofiltration or "NF" is applicable to separate particles ranging from about 10.sup.-3 to 10.sup.-2 microns in size; that is, particles in a size range between those separable by reverse osmosis and ultrafiltration. The instant invention is concerned primarily with reverse osmosis and nanofiltration processes.
Reverse osmosis is described in numerous references such as, e.g., H. K. Lonsdale, "Reverse Osmosis," in "Synthetic Membranes, Science, Engineering and Applications," Ed. by Bungay, Lonsdale and dePinho, D. Reidel Publ. Co., Boston, Mass., 1986, pp 307-342. Cellulose acetate is an example of a typical RO membrane material.
Osmosis takes place when two different solutions are separated by an appropriate semipermeable membrane. The osmotic pressure across the membrane is directly proportional to the solute concentration difference between the two solutions, and the manifestation of osmotic pressure is diffusion of solvent from the more dilute solution, through the membrane, and into the more concentrated solution. Reverse osmosis, as the name implies, requires the application of external pressure to the more concentrated solution sufficient to overcome the osmotic pressure.
The result of such applied pressure, which can be as high as about 10.sup.3 psi, is to transfer solvent from the more concentrated solution, across the membrane and into the less concentrated solution; that is, reverse osmosis tends to produce a purified solvent stream on one side of the membrane and a more concentrated solution of the solute on the other side. In reverse osmosis, the solvent flux is directly proportional to the pressure in excess of the osmotic pressure which is applied to the solution, and the flux ultimately is limited on the high side by the mechanical capabilities of the equipment and/or membrane.
Reverse osmosis has been and is being employed around the world to produce potable drinking water from seawater and brackish waters. Seawater contains about 2-3 wt % total dissolved solids, about 50-100 ppm of which is bromide. Brackish water typically contains about ten-fold less total dissolved solids. RO is also used in the electronics industry to make ultra pure water for various processes, and RO is employed to clean industrial effluents. Obtaining the purified solvent resulting from RO has usually been the objective, and scant attention has been paid to any possible values derived from the concentrated solute solution produced on the other side of the membrane.
One instance in which the objective of the membrane process is the production of a concentrated solute solution is the treatment of cheese whey to concentrate it. At the same time, the whey is desalted. See A. G. Gregory, "Desalination of Sweet-Type Whey Salt Drippings for Whey Solids Recovery," Associated Milk Producers, Inc., North Central Region, Bulletin of the International Dairy Federation #212, 1987.
Nanofiltration, also called "loose reverse osmosis," employs a semipermeable membrane through which some solutes retained by a reverse osmosis membrane can readily pass; see, e.g., Desalination, 70, 77-88 (1988). Nanofiltration does not require pressures as high as those associated with reverse osmosis; pressures of about 10.sup.2 psi often suffice for nanofiltration, and such membranes can, as with RO membranes, be employed in water purification as disclosed in U.S. Pat. No. 4,981,594.
Certain nanofiltration membranes have the ability to discriminate between, not only particles of different size, but also particles of different charge sign and magnitude which will pass through the NF membrane. This discrimination can be explained in terms of the Donnan exclusion model; see Bhattacharyya, et al., Prog. Clin. Biol. Res., 292, 153-167 (1989), for example.
The NF membranes of particular interest herein are membranes that discriminate between anions of different charge. Such membranes are described, for example, in U.S. Pat. No. 4,872,991 and can comprise the condensation product of a polyfunctional amine and a polyfunctional carboxylic component, i.e., a polyamide, which carries pendant --COO. groups. A commercially available membrane of this type is the NF-40 membrane available from FilmTec Corporation, Minneapolis, Minn. According to the Donnan model, the pendant carboxylate units on the surface of such a membrane repel those solute anions which diffuse to the membrane, and the degree of repulsion increases with increasing charge density of the anion, anions of higher charge density being repelled more strongly. This makes it possible for a nanofiltration unit to separate an aqueous stream containing NaCl and Na.sub.2 CO.sub.3 into two separate streams, one enriched in NaCl and the other in Na.sub.2 CO.sub.3, for example.
This ability of NF to separate anions by charge magnitude has been employed in the recovery of oil by injecting water into the oil-bearing formation, i.e., U.S. Pat. No. 4,723,603. The injection water is pretreated by NF to reject SO.sub.4.sup.-2 in the retentate and pass monovalent anions in the permeate, which is sent to the well, thus curtailing the precipitation of metal sulfates such as BaSO.sub.4, CaSO.sub.4, etc. downhole, which could plug the passageways in the formation. U.S. Pat. No. 4,806,244 discloses a combined membrane/sorption process in which an NF membrane is used to separate SO.sub.4.sup.-2 and NO.sub.3.sup.-1 in a water stream. Similar applications for nanofiltration are described in Environmental Progress, 7, 58-62 (1988). Although the NF membrane process succeeds in separating monovalent anion from polyvalent anion, the NF membrane permeate generally is not as concentrated in monovalent anion as the feed to the membrane.
Several hybrid systems which combine either reverse osmosis or nanofiltration with a second separation process, such as pervaporation, coupled transport, etc. are described in U.S. Pat. No. 4,944,882. Combinations of reverse osmosis with another membrane process, i.e., ultrafiltration, are described in Desalination, 47, 257-265 (1983) and in Tappi Journal, 69, 122-125 (1986).
Processes employed in the chemical industry offer many opportunities for the use of semipermeable membrane technology in various purifications and related separations. A portion of the chemical industry is devoted to the recovery of elemental bromine from subterranean bromide-containing brines and in the manufacture and sale of brominated flame retardant additives for use in protecting flammable materials such as fabrics and plastics. Such flame retardant additives include, for example, decabromodiphenyl oxide and tetrabromobisphenol A.
A number of the brominated organic flame retardants are made by reacting an organic starting material with elemental bromine, producing bromides such as sodium bromide and hydrobromic acid as byproducts. These bromide-containing byproducts represent bromine values which have been difficult to recover and often are simply disposed of as waste by deep well injection. This bromide-containing waste could be recycled into an elemental bromine recovery process, e.g., as described in U.S. Pat. No. 4,978,518, except that these byproduct streams generally contain considerable amounts of soluble sulfate, sulfite, carbonate, phosphate and phosphite which, when combined with a subterranean brine, lead to intractable scale deposits in the processing equipment. The subterranean bromide brines generally are loaded with barium, calcium and strontium cations, whose sulfates, carbonates and phosphates are virtually insoluble in water.